This thesis investigates a new generation of collaborative systems: tabletop tangible interfaces (TTIs) for music performance or musical tabletops. Musical tabletops are designed for professional musical performance, as well as for casual interaction in public settings. These systems support co-located collaboration, offered by a shared interface. However, we still know little about their challenges and opportunities for collaborative musical practice: in particular, how to best support beginners or experts or both. This thesis explores the nature of collaboration on TTIs for music performance between beginners, experts, or both. Empirical work was done in two stages: 1) an exploratory stage; and 2) an experimental stage. In the exploratory stage we studied the Reactable, a commercial musical tabletop designed for beginners and experts. In particular, we explored its use in two environments: a multi-session study with expert musicians in a casual lab setting; and a field study with casual visitors in a science centre. In the experimental stage we conducted a controlled experiment for mixed groups using a bespoke musical tabletop interface, SoundXY4. The design of this study was informed by the previous stage about a need to support better real-time awareness of the group activity (workspace awareness) in early interactions. For the three studies, groups musical improvisation was video-captured unobtrusively with the aim of understanding natural uses during group musical practice. Rich video data was carefully analysed focusing on the nature of social interaction and how workspace awareness was manifested. The findings suggest that musical tabletops can support peer learning during multiple sessions; fluid between-group social interaction in public settings; and a democratic and ecological approach to music performance. The findings also point to how workspace awareness can be enhanced in early interactions with TTIs using auditory feedback with ambisonics spatialisation. The thesis concludes with theoretical, methodological, and practical implications for future research in New Interfaces for Musical Expression (NIME), tabletop studies, and Human-Computer Interaction (HCI).